Photovoltaic power generation is in the forefront of rapidly growing industries, scoring high growth rates globally. In the US, PV installations grew 109% in 2011 to reach 1,855 megawatts (MW) which represents 7.0% of all PV globally. PV installations can be divided between three market segments, namely residential, commercial and utility. Utility installations typically exhibit the largest growth among these market segments. This significant growth is encouraged by the continuous drop in PV costs.
PV modules are generally characterized by their nonlinear voltage-current (I-V) relation that is significantly affected by external environmental factors like solar irradiance and temperature, such as illustrated in the graph 600a of FIG. 6A, in conjunction with other factors like aging and module mismatch. Among these factors, shading is usually the most critical player affecting energy harvest from PV modules. As depicted in the graph 600a in FIG. 6A, the PV module current significantly drops at low irradiance levels that can be caused by shading. Maximum power is obtained only at a specific value of current for a given irradiance, as indicated from FIG. 6A. It is a purpose of the Maximum Power Point Tracking (MPPT) controller to harvest a maximum power from the PV module for any of various given environmental conditions.
Most PV modules deliver power at low voltages, typically 25-35 volts for most crystalline silicon modules and 50-100 volts for most thin-film modules. Therefore, it is generally required to build up an acceptable voltage level either by series connection of PV modules, or by voltage-boosting of parallel connected modules via a power electronic stage that can raise the system cost and can reduce its efficiency. Thus, constructing strings of series connected modules is the dominant design choice in most PV systems especially for commercial and utility scales. These strings can be grouped in parallel to one large central inverter, or each string can be connected to a string inverter.
Since the inverter efficiency generally increases with its power rating, central inverters can be more efficient in power conversion than all other alternatives, i.e. string inverters (96% efficiency) and Module Integrated Converters (MIC) (94% efficiency), for example. Additionally, the inverter cost per kilowatt (kW) generally decreases with increasing the inverter size and, hence, a central inverter can offers the relatively cheapest solution. Conversely, from the perspective of the amount of extracted output power from a given PV plant, due to modules mismatch and environmental effects like partial shading, central inverters typically extract less energy compared to string inverters and MIC, respectively. Nevertheless, central inverters still prove to be a dominant candidate for large PV installations as to its relatively better gross economics, despite the technological competition of string inverters.
In large PV power plants, PV modules are typically connected in series to create strings with the desired peak direct current (DC) voltage. As previously mentioned, these strings are usually connected in parallel and fed to one large central grid-connected inverter that converts DC to alternating current (AC), such as illustrated in FIG. 5, for example. FIG. 5 is a schematic diagram illustrating a power generation system 500 including photovoltaic (PV) arrays of a plurality of DC voltage source strings, such as DC voltage source string 510 and 520. The DC voltage source string 510 includes a plurality of PV panels 512 each panel being associated with a bypass diode 515 and including a reverse blocking diode 518. The DC voltage source string 520 includes a plurality of PV panels 522 each panel being associated with a bypass diode 525 and including a reverse blocking diode 528. A central inverter 530 receives the DC voltage generated by the DC voltage source strings 510 and 520 and converts the received DC voltage to an alternating current (AC) voltage, for example.
A series connection of modules generally dictates the flowing of the same or substantially the same current in all modules. Nevertheless, if a module is shaded, its current providing capacity can be reduced and, in such case, it is usually protected from the high current of the series unshaded modules through a bypass diode. For a partially shaded string, a decision to work at one of two operating conditions can be made. For example, a foremost operating condition of the partially shaded string is to let the string perform at the level of the poorest performer in the string, i.e. the low current of the shaded module dictates the current of the whole string so that all modules are producing power, i.e. no modules are shorted by their bypass diodes and, thus, the string voltage does not collapse. A second operating condition of the partially shaded string is to operate the string at the relatively high current of the unshaded modules and sacrificing the shaded ones, i.e. the shaded modules are shorted by their bypass diodes. From the overall string power perspective, this second operating condition option is usually better than operating the whole string at low current of the shaded modules.
Referring to FIG. 6B, in a graph 600b of this figure there is illustrated the second operating condition and shows that a maximum power of a DC voltage source string is obtained at lower string voltage, i.e. at higher string current. In this regard, the graph 600b of FIG. 6B shows, for example, the power-voltage relation of a string of 20 series panels with different numbers of shaded panels that receive a reduced irradiance of S=200 watts/meter2 (W/m2), and the unshaded panels receive the rated irradiance of S=1000 W/m2.
Unfortunately, a partially shaded string operating at its maximum power point with a reduced voltage typically fails to connect in parallel with the unshaded strings operating at a higher voltage. Such parallel connection usually can result in one of two unwanted conditions, namely either operating the parallel strings at a higher voltage assuring maximum power yield from the unshaded strings and sacrificing a significant amount of available power from the partially shaded string or operating the parallel strings at lower voltage to assure a maximum power yield from the shaded string and sacrificing a significant amount of available, but unutilized, power from the unshaded strings. Logically, typically the unshaded strings tip the scale towards operating at a higher voltage to maximize the overall power from the PV plant.
As seen in the graph 600b of FIG. 6B, parallel connection of a partially shaded string to other strings generally results in reduced extracted output power from the partially shaded string if operated at a maximum power point (MPP) voltage of the parallel non-shaded strings. FIG. 6C shows in a graph 600c the percentage of unutilized power from the shaded string when operating at the voltage corresponding to a maximum power of the unshaded strings. The curves are at different insolation values (S=0, 200, 400, 600 W/m2) for the shaded modules and at different percentages of shaded modules per string.
For various PV modules from different manufacturers, the voltage at maximum power point is typically about 80% of the open circuit voltage of a PV module. Consequently, it can be noted that, when about 20% of a string of a series PV modules is completely shaded, the shaded string voltage is usually below the maximum power point voltage (VMPP) of the unshaded strings, such that VOpen Circuit20% shaded string<VMPPun-shaded string, for example. Thus, no power from the shaded string usually can be harvested at this voltage and 100% of the available power of the shaded string is lost, such as indicated by the graph 600c of FIG. 6C, for example. Traditionally, in order to protect a shaded string from a reverse current caused by this voltage difference each string is serially connected with a blocking diode. Table 1 below shows for different PV manufactures that VMPP/VOpen Circuit≈80%, for example.
TABLE 1MANUFACTURERS' DATA OF SOME PV MODULES:100% * (1 − VMPP/VOpenModuleVMPPVOpen CircuitCircuit)Sharp 175 W35.444.420.3%First Solar 75 W68.289.623.9%Signet 360 W146.4187.6  22%Thus, from the above description and Table 1, it can be explained and understood as to why a complete shading of 20% of the string can lead to 100% unutilized power from the partially shaded strings, as is also indicated from the illustration of FIG. 6C of the unutilized power in a partially shaded string of DC voltage sources, such as including PV panels, for example.
While current available solutions can assist in overcoming low energy yield of central inverters, typically such solutions are at the expense of decreased conversion efficiency and increased system cost. For example, while a string inverter can be one viable alternative, it is not necessarily an economical solution. Another possible alternative is using a DC-DC converter (a boost converter, for example) for each string to manipulate voltages of different strings independently to maximize the power from each of the different strings. However, use of such DC-DC converter typically can require the DC-DC converter to be rated with the full DC bus voltage and full string power, with a consequently relatively higher system cost and increased power losses can also be introduced.
It would therefore be desirable to have a balancing circuit topology, as well as an extracted output power optimizer circuit topology to optimize extracted output power from a corresponding string, and control that can integrate with conventional central-inverter-based PV installations in order to increase their energy harvest. Additionally, it would be desirable for such balancing circuit topology to have a relatively small power rating of power electronic switches and passive elements, to provide a relatively lost cost, as well as have the ability to minimize power losses and have a relatively long lifetime operation.
Thus, apparatuses and methods for a voltage balancing topology coupled with renewable energy sources using maximum power point tracking and control of power generation addressing the aforementioned problems is desired.